1. Field of the Invention
This invention relates to the field of ultrasonic transducers, and more particularly, to the field of phased array ultrasonic transducers.
2. Background Information
Array transducers, whether they be ultrasonic transducers as in the case of ultrasonic imaging, or electromagnetic radiating horns as in the case of phased array radars, rely on wave interference for their beam forming effects. The ability to provide a focused beam on transmission and to provide a clear image on reception is dependent on each of the elements of the array having identical transduction characteristics between the electrical signals provided by the system transmitter and the wave transmitted into the medium to be explored and identical transduction functions from a wave in the medium being explored to an electrical signal provided to the signal processing system. It is only when the elements have identical characteristics that phased array combining of the signals from a plurality of elements will provide a clear image. The element characteristics which is used to compare elements is the element impulse response. That is, the element's response when a brief high amplitude electrical or wave pulse is applied to the element.
It is because of this theoretical basis for phased array processing, imaging, and coherent beam forming that phased arrays are fabricated from a plurality of elements having identical impulse responses. Since large and small objects react differently, the prior art has satisfied this requirement by using physically identical transducers in order to provide identical impulse responses.
Initially, ultrasonic transducers were individual, stand alone transducers. For imaging and surveillance purposes, linear arrays of ultrasonic transducers and two-dimensional arrays of ultrasonic transducers were developed, along with appropriate electronics, to provide images of objects whose characteristics it was desired to determine. Early two-dimensional ultrasonic arrays were relatively large structures in which individual, identical elements of the array were separately fabricated and then assembled into an array which was suitable for use in such large scale systems as sonar.
In such arrays, individual elements had a height of a wavelength or more. In this specification, as will be discussed subsequently in greater detail, the thickness of a piezoelectric array element is defined as being perpendicular to the face of the array, the width of an array element is defined as the narrow dimension of the element which is disposed parallel to the face of the array and the height of the element is defined as the long dimension of the element which is disposed parallel to the face of the array.
In elements having a width that is in the vicinity of a wavelength of longer, the thickness acoustic vibrations of the piezoelectric element and the width vibrations of the piezoelectric material couple to each other resulting in undesirable piezoelectric transducer characteristics. In prior art linear arrays of this type, it was found that this coupling between the thickness and width modes of the acoustic vibrations in the piezoelectric material could be suppressed by subdicing the elements of the array into segments of piezoelectric material in which each segment of the piezoelectric material is the same size and with a maximum width on the order of half the thickness. Consequently, such linear arrays are normally subdiced to improve their electro-acoustic characteristics. By subdicing, we mean cutting most of the way through the piezoelectric material, preferably without going all the way through it. This separates the piezoelectric into acoustically separate segments, while preferably leaving it as a unitary structure. The separate segments of an element have their signal electrodes connected together in order to function as a single electrical element.
When interest developed in the use of ultrasound as a medical imaging tool, much smaller arrays and elements were required than were used in prior art ultrasonic phased arrays.
There are two different kinds of ultrasonic imagers which use linear transducer arrays. The first is a rectilinear scanner in which a subarray consisting of a specified number of elements is selected and focused, usually without steering, i.e. with the beam direction perpendicular to the plane of the array face. An electrical signal is applied to each of the elements of this subarray to induce the transmission of a beam of ultrasound into the object to be examined and the reflection of that beam is received by the same subarray and converted to electrical signals which contribute to the generation of an image. A new subarray is then selected and the process repeated until the desired rectangular image can be generated. Typically, successive subarrays of N elements each have N-1 elements in common such that each successive subarray drops one element from the previous subarray while adding the next element in the array. Typically, these transducer elements have widths which are greater than .lambda. in the object to be examined and are subdiced as described in the previous paragraph to obtain desired element response characteristics.
The second kind of linear array is a phased array sector scanner in which all of the transducer elements are used simultaneously to form a steered beam. In this type of array, the individual element widths have to be small (.about..lambda./2 in water) in order for the beam formation process to be effective. It is linear arrays of this second kind which are most similar to the two dimensional phased arrays to which the present invention is directed.
Medical ultrasonic arrays are typically linear arrays of elements formed from a single block of piezoelectric material which is appropriately processed to produce an array of physically connected, but electrically substantially independent, acoustic transducers. Each of these transducers is separately connected to the system electronics either for generation of sound for transmission into the body to be examined or for reception of sound from the body being examined, or both.
As the diagnostic use of ultrasound has progressed, a need has developed for greater resolution and image clarity. Typical medical linear acoustic phased array transducers have elements that are small enough that coupling between the thickness and the width modes of the acoustic vibrations in the piezoelectric material are not a problem.
In typical prior art linear acoustic phased array transducers for medical purposes, the array has narrow, closely spaced elements disposed along its X-direction length which are capable of focusing the acoustic beam in the X-direction at a particular depth and/or steering the acoustic beam to a particular location in the X-direction (along the length of the linear array). However, perpendicular to the length of the linear array (Y-direction), focus was provided by a fixed acoustic lens having a fixed focal depth with the result that focusing the linear array at a substantially shallower or substantially greater depth resulted in a lack of focus in the Y-direction. No Y-direction steering is provided.
Related U.S. Pat. No. 4,890,268 overcame this Y-direction focus problem by providing a two-dimensional acoustic array transducer of medical dimensions which is capable of focusing a 5 MHz acoustic beam in the desired manner in both directions, while steering it in the X-direction. The two-dimensional array of that patent is an approximation to a circular Fresnel lens. As such, it may be looked upon as being formed of a plurality of linear X-direction acoustic phased array transducers stacked in the Y-direction. As is illustrated in FIG. 1, in order to form an accurate approximation to a circular Fresnel lens, the individual subarrays have differing heights in the Y-direction. In accordance with phased array theory, this structure would have unusable because different subarrays would have had different element impulse responses since the patent uses elements which vary by more than 3 to 1 in size.
U.S. Pat. No. 4,890,268 avoids the problem of differing impulse responses in the elements of the different subarrays by forming each of the elements from a plurality of uniform width piezoelectric segments in which all dimensions except the thickness dimension are less than about half a wavelength. This is accomplished by forming that transducer from a 2--2 composite of piezoelectric slabs and electro-acoustically inert slabs. A 2--2 composite is one in which the material of each of its two components is connected to itself over large distances in only two perpendicular directions. That is, the structure from which that array is formed is essentially a laminate of multiple piezoelectric slabs interleaved with multiple slabs of an acoustically inactive material such as epoxy. The transducer is then formed by subdicing and dicing this laminate structure to produce the desired pattern of array elements. The impulse response of each element is determined by the impulse response of the individual piezoelectric segments. Thus, U.S. Pat. No. 4,890,268 follows the prior art pattern of using "identical" elements by incorporating a plurality of physically identical piezoelectric segments in each of its electrical elements in order that the impulse response of all the elements will be identical, despite their differing physical size. While this structure is precise in providing identical impulse responses for all of the electrical elements, it is complex and relatively expensive to manufacture. A transducer structure retaining the benefits of U.S. Pat. No. 4,890,268 array structure while simplifying the manufacturing process and reducing the manufacturing cost would be highly desirable.